Rigid polyurethane foams and rigid polyisocyanurate foams (hereinafter collectively referred to as “rigid foamed synthetic resins”) are widely produced by reacting a polyol component with a polyisocyanate component in the presence of a foam stabilizer, a catalyst and a blowing agent. By virtues of their good heat insulating properties and molding versatility, rigid foamed synthetic resins are suitably used as heat insulators for various apparatus and buildings. These rigid foamed synthetic resins are produced by spraying, slab stock molding, injection molding or other molding methods.
For example, spraying is mostly employed to produce rigid foamed synthetic resins as heat insulators at construction sites. In spraying, a polyol system solution containing a polyol, a blowing agent and the like and a polyisocyanate compound are pumped separately and sprayed from a spray gun onto walls and other surfaces so that they react to form a foam on them. An advantage of spraying is that walls and other surfaces of any shape can be covered with insulators of a desired thickness. Among the spraying techniques, multilayer spraying can form a thick heat insulating barrier with good heat insulating properties by spraying a rigid foamed synthetic resin twice or more in layers.
In general, rigid foamed synthetic resins having lower densities are desired because they would be produced from smaller amounts of raw materials and, hence, at lower cost and they would be light enough to be transported easily. However, as the density decreases, they develop a problem with dimensional stability and become mechanically weak and likely to shrink. Therefore, it is necessary to develop low density rigid foamed synthetic resins with excellent mechanical strength and dimensional stability.
From the standpoint of fire prevention, as a building material, rigid foamed synthetic resins are required to have flame retardancy. Especially, when they are formed by spraying, flame retardancy is required in order to prevent welding sparks at construction sites from starting a fire.
At present, for production of rigid foamed synthetic resins, as blowing agents, hydrofluorocarbons (such as HFC-245fa, HFC-365mfc, hereinafter referred to as HFCs) are mainly used. However, considering their adverse impact on the environment, it is desired to reduce the use of HFCs. Therefore, techniques for reducing the use of HFCs have been explored by using water as a blowing agent to compensate for the reduction of HFCs. However, because the foam density attained with a HFC can be attained with much less water, there is a problem that if water is used instead of an HFC at the sacrifice of the solvency of the HFC, a polyol, a blowing agent, a foam stabilizer, a catalyst and other necessary additives form a viscous polyol system solution which adversely affects moldability and workability.
Besides, there is another problem that as the water content in the polyol system solution increases, the polyol system solution becomes less miscible with a highly hydrophobic polyisocyanate compound, and molding defects tend to occur. Especially, the above-mentioned problems with the use of water as a blowing agent are remarkable in spray molding, and there is a strong demand that these problems be solved at the same time as achievement of long-sought reduction of densities of rigid foamed synthetic resins and better flame retardancy.
Meanwhile, polyether polyols obtained from the products of Mannich condensation reaction of phenols, aldehydes and alkanolamines are advantageous in terms of mechanical strength and flame retardancy and highly evaluated.
Patent Documents 1 to 5 mentioned below disclose Mannich polyols obtained by using Mannnich condensation products from phenols, aldehydes and alkanolamines.
Patent Document 1 relates to a process for producing a rigid polyurethane foam using water as a blowing agent and discloses that it is preferred to react a phenol, an aldehyde and an alkanolamine in a phenol:aldehyde ratio of from 1:1 to 1:2.2.
Patent Document 2 discloses a Mannich polyol obtained by addition of an alkylene oxide to a Mannich condensation product with a high functionality obtained by mixing 1 mol of a phenol with from 1 to 2 mol of an amine compound and then adding from 1.25 to 1.75 mol of formaldehyde.
Patent Document 3 relates to a process for producing a rigid foamed synthetic resin and discloses a low-viscosity Mannich polyol obtained from a Mannich condensation product with high functionality obtained by condensing an alkylphenol, diethanolamine and formaldehyde in a ratio of 1:2.5-4:1.5-2.
Patent Document 4 relates to a process for producing a Mannich polyol by reacting a phenolic compound, formaldehyde and an alkanolamine and dehydrating the resulting Mannich condensation product at least partially and alkylating the dehydration product and discloses that the phenolic compound and formaldehyde are used preferably in a molar ratio of 1:0.9-3.5.
Patent Document 5 relates to a process for producing a polyether polyol by adding propylene oxide and/or butylene oxide (but no ethylene oxide) to the reaction product of a phenol, an aldehyde and an alkanolamine and discloses that the aldehyde is used in an amount of from 1.35 to 3.0 mol in relation to 1 mol of the phenol.